The present invention relates generally to magnetic resonance (MR) imaging and, more particularly, to a method and apparatus that enables a user to interactively set parameters of an MR pulse sequence through inspection of frequency spectra derived from pre-scan data. In particular, the invention is directed to the acquisition of non-phase encoded data that is acquired in a readout window without a readout gradient.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
The principles of MR have been exploited to develop numerous imaging protocols to capture contrast from static tissue as well as flowing blood. One commonly implemented imaging option that is used to improve image contrast is chemical saturation. With chemical saturation data is collected for an image from just one chemical shift component in a region-of-interest. This is achieved by suppressing the NMR signals from those hydrogen nuclei that correspond to the chemical shift component that is not to be imaged. For example, if the region-of-interest is primarily composed of water and fat, each having a different chemical shift, through application of the appropriate chemical saturation, data collected and the resulting image would be of either water or fat, but not both. As the nuclei of fat are normally suppressed, chemical suppression is generally referred to as fat suppression.
In conventional fat suppression, a frequency selective saturation pulse is applied before standard imaging pulses of the pulse sequence. The frequency selective saturation pulse suppresses the magnetization from fat. Accordingly, the reconstructed image consists mainly from water magnetization.
For fat and other chemical saturations to be effective, the frequency of the frequency selective saturation pulse must correspond to the resonant frequency of the component to be suppressed. While general guidance has been empirically developed for setting the frequency and amplitude of the saturation pulse, ultimately, the nuances of the MR scanner and the particulars of the object to be scanned generally necessitate a pre-scan to assist a user in determining the most appropriate parameters for the saturation pulse and other parameters of an impending scan. This fine tuning then allows the user to define the pulse sequence that will result in optimal fat signal suppression and optimal image quality.
In manual fine tuning of the preparatory (saturation) pulse (and imaging pulses), the user interactively sets the center frequency of the component to be imaged, typically water, the offset or chemical shifted frequency of the component to be suppressed, typically fat, and the amplitude of the RF pulse that will be applied to minimize signal emissions from the component to be suppressed. The interactive adjustment is made by generating a frequency spectrum from MR data acquired in a pre-scan which allows a user to visualize the signal from water and fat. It has been found that the amplitude for the saturation pulse that optimally minimizes fat signal is dependent upon the repetition time (TR) of the RF saturation pulse, the spin-lattice relaxation time (T1) of fat, and the imaging pulses that also “flip” the component to be saturated. However, in conventional manual fine tuning of the saturation pulse amplitude and/or offset frequency, the RF pulse sequence used during the pre-scan has a TR that is typically unrelated to the effective TR of the saturation pulse for the clinical scan. In particular, the effective TR of the saturation pulse corresponds to the chosen clinical imaging TR divided by the number of slices excited during this TR interval. In circumstances where the TRs differ significantly, the amplitude of the saturation pulse that visually minimizes the fat signal in the pre-scan may not minimize the fat signal during the imaging scan. As a result, image quality is less than expected because saturation is less than expected, and may lead to repeated scans.
Similar drawbacks have been discovered in other pre-scan pulse sequences. That is, in some known pre-scans, pre-scan data is acquired under conditions that do not accurately reflect the conditions in which imaging or clinical data is acquired, leading to suboptimal determination/setting of the parameter(s) of interest. Moreover, for some systems and/or scans, interactive optimization is sometimes not performed or allowed at all which can result in repeated and time consuming scans.
It would therefore be desirable to have a system and method capable of enabling a user to interactively determine and set parameters for a imaging sequence with the aid of pre-scan data that is collected with a pre-scan pulse sequence such that the relative and/or absolute levels of MR signals in a frequency spectrum generated from the pre-scan data match those expected with an imaging scan. Such a system and method would enable a user to optimally define parameters for an impending scan conveniently, expeditiously, and efficiently.